U.S. patent number 5,254,411 [Application Number 07/803,486] was granted by the patent office on 1993-10-19 for formation of heat-resistant dielectric coatings.
This patent grant is currently assigned to Shin-Etsu Chemical Co., Ltd.. Invention is credited to Toshinobu Ishihara, Kenichi Ito, Yoshihumi Takeda.
United States Patent |
5,254,411 |
Takeda , et al. |
October 19, 1993 |
Formation of heat-resistant dielectric coatings
Abstract
Heat resistant, dielectric coatings are formed by applying a
heat resistant coating composition comprising an organic silicon
polymer, a silazane compound, and an inorganic filler to a
substrate, and baking the coating in ammoniacal atmosphere at
200.degree. to 1000.degree. C. Similarly, heat resistant,
dielectric coatings are formed by applying the same composition as
above to a substrate, baking a first coating layer in air, applying
an organic silicon polymer base coating composition to the first
coating layer, and baking a second coating layer in ammoniacal
atmosphere.
Inventors: |
Takeda; Yoshihumi (Niigata,
JP), Ishihara; Toshinobu (Niigata, JP),
Ito; Kenichi (Niigata, JP) |
Assignee: |
Shin-Etsu Chemical Co., Ltd.
(Tokyo, JP)
|
Family
ID: |
26582940 |
Appl.
No.: |
07/803,486 |
Filed: |
December 4, 1991 |
Foreign Application Priority Data
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Dec 5, 1990 [JP] |
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2-405377 |
Dec 5, 1990 [JP] |
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2-405378 |
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Current U.S.
Class: |
428/447; 427/377;
427/387 |
Current CPC
Class: |
B05D
3/04 (20130101); C04B 41/009 (20130101); C04B
41/5066 (20130101); C04B 41/52 (20130101); C04B
41/87 (20130101); C04B 41/89 (20130101); C23C
18/1212 (20130101); C23C 18/122 (20130101); C23C
18/1225 (20130101); C23C 18/1275 (20130101); C09D
183/16 (20130101); C04B 41/5066 (20130101); C04B
41/4519 (20130101); C04B 41/4554 (20130101); C04B
41/5025 (20130101); C04B 41/52 (20130101); C04B
41/4517 (20130101); C04B 41/4554 (20130101); C04B
41/5025 (20130101); C04B 41/5066 (20130101); C04B
41/52 (20130101); C04B 41/4517 (20130101); C04B
41/4554 (20130101); C04B 41/5066 (20130101); C04B
41/009 (20130101); C04B 35/00 (20130101); Y10T
428/31663 (20150401) |
Current International
Class: |
B05D
3/04 (20060101); C04B 41/45 (20060101); C04B
41/52 (20060101); C04B 41/89 (20060101); C04B
41/87 (20060101); C04B 41/50 (20060101); C09D
183/16 (20060101); C23C 18/12 (20060101); C23C
18/00 (20060101); B32B 009/04 () |
Field of
Search: |
;427/377,387 |
Foreign Patent Documents
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337618 |
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Oct 1989 |
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EP |
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372381 |
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Jun 1990 |
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EP |
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377118 |
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Jul 1990 |
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EP |
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234069 |
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Aug 1988 |
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JP |
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2-58580 |
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Feb 1990 |
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JP |
|
2092534 |
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Apr 1990 |
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JP |
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Other References
Patent Abstracts of Japan, vol. 14, No. 291 (M-989) Jun. 22, 1990
and JP A-2,092,534 (Showa Electric Wire & Cable Co. Ltd.) Apr.
3, 1990. .
Patent Abstracts of Japan, vol. 13, No. 29 (C-562) (3377) Jan. 23,
1989 and JP A-63-234 069 (Showa Electric Wire & Cable Co.,
Ltd.) Aug. 29, 1988. .
Chemical Abstracts, vol. 100, No. 20, May 1984, Columbus Ohio, US;
Abstr. No. 160896H, Kazakov M. E. et al., Thermal. Transformations
of Methyl Phenyl Oligocyclosilazanes on a Carbon Susbstrate. .
Chemical Abstracts vol. 90, No. 14, Apr. 1979, Columbus Ohio, U.S.;
Abs. No. 108623C, Mazaev V. A. Material Made from Polymer Saturated
Boron Nitride..
|
Primary Examiner: Lusignan; Michael
Attorney, Agent or Firm: McAulay Fisher Nissen Goldberg
& Kiel
Claims
We claim:
1. A method for forming a heat resistant, dielectric coating
comprising the steps of
applying a heat resistant coating composition comprising an organic
silicon polymer, a silazane compound selected from the group
consisting of tetramethyldisilazane, hexamethylcyclotrisilazane,
octamethylcyclotetrasilazane and mixtures thereof, and an inorganic
filler to a substrate, and
baking the coating in an atmosphere of ammonia gas or a mixture of
ammonia gas and an inert gas.
2. The method of claim 1 wherein said coating composition contains
about 5 to about 40% by weight of silazane compound and about 10 to
about 70% by weight of inorganic filler, the remainder being
organic silicon polymer.
3. The method of claim 1 wherein the baking step is at a
temperature of 200.degree. to 1000.degree. C.
4. The method of claim 1 wherein the organic silicon polymer is
selected from the group consisting of polycarbosilanes and
polysilazanes.
5. A substrate having a heat-resistant dielectric coating thereon,
said coating being prepared by the method of claim 1.
6. A method for forming a heat resistant, dielectric coating
comprising the steps of
applying a first coating composition comprising an organic silicon
polymer, a silazane compound selected from the group consisting of
tetramethyldisilazane, hexamethylcyclotrisilazane,
octamethylcyclotetrasilazane and mixtures thereof, and an inorganic
filler to a substrate,
baking the first composition to form a first coating layer,
applying a second coating composition comprising an organic silicon
polymer to the first coating layer, and
baking the second composition to form a second coating layer on the
first coating layer.
7. The method of claim 6 wherein the step of baking the first
composition is carried out in air at a temperature of at least
about 200.degree. C.
8. The method of claim 6 wherein the step of baking the second
composition is carried out at a temperature of 400.degree. to
800.degree. C.
9. The method of claim 6 wherein the step of baking the second
composition is carried out in an atmosphere of ammonia gas or a
mixture of ammonia gas and an inert gas.
10. The method of claim 6 wherein the organic silicon polymer is
selected from the group consisting of polycarbosilanes and
polysilazanes.
11. The method of claim 6 wherein said coating composition contains
about 5 to about 40% by weight of silazane compound and about 10 to
about 70% by weight of inorganic filler, the remainder being
organic silicon polymer.
12. A substrate having a heat-resistant dielectric coating thereon,
said coating being prepared by the method of claim 6.
Description
This invention relates to a method for forming heat-resistant,
dielectric coatings having improved adhesion, heat resistance, and
electric insulation.
BACKGROUND OF THE INVENTION
Polyorganosiloxane base coating compositions are superior in heat
resistance to coating compositions on organic polymers such as
polyester and polyimide, but cannot withstand elevated temperatures
of higher than 400.degree. C. for a long time.
In the recent years, there is an increasing demand for coating
compositions capable of preventing oxidation and corrosion of
metallic and non-metallic substrates which are serviced at high
temperatures in excess of 1000.degree. C. It is also desired to
develop coating compositions which form coatings maintaining
electric insulation at high temperatures and having good
adhesion.
A variety of heat resistant coating compositions have been proposed
in the art. (1) Japanese Patent Application Kokai (JP-A) No.
54768/1987 discloses a composition comprising
polytitanocarbosilane, silicone resin, and inorganic filler. (2)
JP-A 235370/1987 discloses a composition comprising
polycarbosilane, silicone resin, and inorganic filler. (3) JP-A
92969/1990 discloses a heat resistant coating composition having
organometallic polymer and silicon dioxide blended therein. (4)
Japanese Patent Publication No. 50658/1983 discloses a composition
comprising borosiloxane resin.
These proposals, however, have some drawbacks. Heat resistant
coating compositions (1) and (2) are unsatisfactory in adhesion to
substrates at high temperatures, crack resistance of coatings, and
high-temperature electric insulation. Heat resistant coating
composition (3) suffers from separation and cracking of coatings at
high temperatures and poor electric insulation. Heat resistant
coating composition (4) is poor in water resistance and
high-temperature electric insulation. The previously proposed
approaches do not satisfy all the requirements of high-temperature
adhesion, heat resistance, water resistance, and electric
insulation. There is a need for developing a heat resistant coating
composition capable of satisfying all such requirements.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a
method for forming coatings which firmly adhere to metallic and
non-metallic substrates, and have improved heat resistance, water
resistance, solvent resistance, corrosion resistance, and
high-temperature electric insulation.
The inventors have found that by applying a heat resistant coating
composition comprising an organic silicon polymer, a silazane
compound, and an inorganic filler to a conductive or non-conductive
substrate, and baking the composition in ammonia gas or a mixture
of ammonia and an inert gas, there are formed ceramic
heat-resistant, dielectric coatings on the substrate featuring
improved properties including heat resistance, electric insulation
and close adhesion as well as high hardness, water resistance,
chemical resistance and solvent resistance.
Also the inventors have found that by forming a first coating layer
on a substrate from a heat resistant coating composition comprising
an organic silicon polymer, a silazane compound, and an inorganic
filler, and then forming a second coating layer on the first
coating layer from a coating composition comprising an organic
silicon polymer, especially by baking the second coating layer in
ammonia gas or a mixture of ammonia and an inert gas, there are
formed composite coatings featuring improved properties including
heat resistance, water resistance, close adhesion, solvent
resistance, and electric insulation.
Therefore, in a first form, the present invention provides a method
for forming a heat resistant, dielectric coating comprising the
steps of applying a heat resistant coating composition comprising
an organic silicon polymer, a silazane compound, and an inorganic
filler to a substrate; and baking the coating in an atmosphere of
ammonia gas or a mixture of ammonia gas and an inert gas.
In a second form, the present invention provides a method for
forming a heat resistant, dielectric coating comprising the steps
of applying a heat resistant first coating composition comprising
an organic silicon polymer, a silazane compound, and an inorganic
filler to a substrate; baking the first composition to form a first
coating layer; applying a second coating composition comprising an
organic silicon polymer to the first coating layer; and baking the
second composition to form a second coating layer on the first
coating layer, preferably in an atmosphere of ammonia gas or a
mixture of ammonia and an inert gas.
DETAILED DESCRIPTION OF THE INVENTION
The present invention uses a heat resistant coating composition
comprising an organic silicon polymer, a silazane compound, and an
inorganic filler as essential binding components.
A first essential component of the heat resistant coating
composition according to the present invention is an organic
silicone polymer which is preferably selected from polycarbosilanes
and polysilazanes.
The polycarbosilanes are known from JP-B 26527/1982 (or U.S. Pat.
No. 4,052,430, DE 2618246, FR 2308650 and GB 1551952), for example.
Such polycarbosilanes may be synthesized, for example, by reacting
dimethyldichlorosilane with metallic sodium and subjecting the
resulting polysilanes to pyrolytic polymerization.
The polysilazanes are known from the following patent publications
and applications, all by Shin-Etu Chemical Co., Ltd.
(1) JP-A 290730/1987 which corresponds to U.S. Pat. Nos. 4,771,118
and 4,870,035, FR 2,599,745 and DE 3,719,343 A1 and discloses a
process for manufacturing an organic silazane polymer which
comprises reacting ammonia with a mixture of methyldichlorosilane,
methyltrichlorosilane and dimethyldichlorosilane to obtain an
ammonolysis product, and polymerizing the ammonolysis product in
the presence of a basic catalyst capable of deprotonation to obtain
an organic silazane polymer. Preferably, the mixing ratios of
methyldichlorosilane, methyltrichlorosilane and
dimethyldichlorosilane are in ranges of 55 to 80 mol %, 10 to 30
mol % and 5 to 25 mol %, respectively.
(2) JP-A 117037/1988 and 193930/1988 which correspond to U.S. Pat.
No. 4,869,854, FR 2,606,777 and DE 3,736,914 A1 and discloses a
process for manufacturing an organic silazane polymer which
comprises:
reacting ammonia with a mixture consisting of at least one compound
selected from the group consisting of organic silicon compounds of
the formula (I); ##STR1## at least one compound selected from the
group consisting of organic silicon compounds of the following
formula (II); ##STR2## and at least one compound selected from the
group consisting of organic silicon compounds of the following
formula (III); ##STR3## in which R represents hydrogen, chlorine,
bromine, methyl radical, ethyl radical, phenyl radical or vinyl
radical; R.sub.1 represents hydrogen or methyl radical, R.sub.2
represents hydrogen, methyl radical, ethyl radical, phenyl radical
or vinyl radical and X represents chlorine or bromine, to obtain an
ammonolysis product, the mixing ratios of the organic silicon
compounds shown by the above formulae (I), (II), and (III) being in
ranges of 1 to 25 mol %, 1 to 25 mol %, and 50 to 80 mol %,
respectively, and polymerizing the ammonolysis product in the
presence of a basic catalyst capable of deprotonation to obtain an
organic silazane polymer. Preferably, the amounts of hydrogen,
vinyl radical and alkyl or phenyl radical in R.sub.2 of the organic
silicon compounds of the formulae (II) and (III) are in ranges of
55 to 90 mol %, 0 to 30 mol % and 0 to 30 mol %, respectively.
(3) JP-A 210133/1988 which corresponds to U.S. Pat. No. 4,847,345,
FR 8802317 and DE 3805796 A and discloses a process for
manufacturing an organic silazane polymer which comprises reacting
an organic silicon compound of the following formula (I): ##STR4##
in which R represents hydrogen, chlorine, bromine, methyl radical,
ethyl radical, phenyl radical or vinyl radical, R.sub.1 represents
hydrogen or methyl radical and X represents chlorine or bromine, or
a mixture of an organic silicon compound of the formula (I) above
and an organic silicon compound of the following formula (II):
##STR5## in which R.sub.2 and R.sub.3 represent hydrogen, chlorine,
bromine, methyl radical, ethyl radical, phenyl radical or vinyl
radial and X represents chlorine or bromine with a disilazane of
the following formula (III): ##STR6## in which R.sub.4, R.sub.5,
R.sub.6 represents hydrogen, methyl radical, ethyl radical, phenyl
radical or vinyl radical in an anhydrous state at a temperature of
from 25.degree. C. to 350.degree. C. while distilling off
by-produced organic ingredients out of the system to obtain an
organic silazane polymer.
Preferably, the mixing ratio of the organic silicon compounds shown
by the above formulae (I) and (II) is in the range of 50 to 100 mol
%: 0 to 50 mol %.
(4) JP-A 153730/1989 which discloses a method for preparing an
organic silazane polymer comprising the steps of: reacting ammonia
with a mixture of an organic silicon compound of the following
formula (I): ##STR7## in which R represents methyl radical, ethyl
radical or phenyl radical and X represents chlorine or bromine, and
an organic silicon compound of the following formula (II): ##STR8##
in which R represents methyl radical, ethyl radical or phenyl
radical, R.sub.1 represents hydrogen or vinyl radial and X
represents chlorine or bromine, in a mixing ratio of the compounds
(I) and (II) ranging from 20:80 to 90:10 (mol %) to obtain a
silazane compound, and polymerizing the silazane compound in the
presence of an alkali catalyst to obtain an organic silazane
polymer.
(5) JP-A 50238/1991, 51315/1991 and 51316/1991 which correspond to
U.S. Ser. No. 07/554,129 and EP 409146 A2 and disclose a method for
preparing an organic silazane polymer, comprising the steps of:
passing a silazane compound in vapor form through a hollow tube
heated at a temperature in the range of from 400.degree. to
700.degree. C. for activating the silazane compound, and
thermally polymerizing the silazane compound in a liquid phase.
Preferably the silazane compound has the following formula (I) or
(II): ##STR9##
(6) JP-A 81330/1991 which corresponds to U.S. Ser. No. 07/571,132
and EP 417562 A2 and discloses a method for preparing an
polytitanocarbosilazane polymer comprising the step of reacting
(A) an organic silicon compound of the general formula: ##STR10##
wherein R is selected from the group consisting of hydrogen,
chloro, bromo, methyl, ethyl, phenyl and vinyl radicals, R.sup.1 is
hydrogen or a methyl radical, and X is chloro or bromo,
(B) an organic silicon compound of the general formula: ##STR11##
wherein R.sup.2 and R.sup.3 are independently selected from the
group consisting of hydrogen, chloro, bromo, methyl, ethyl, phenyl
and vinyl radicals, and X is chloro or bromo,
(C) a titanium compound, and
(D) a disilazane of the general formula: ##STR12## wherein R.sup.4,
R.sup.5 and R.sup.6 are independently selected from the group
consisting of hydrogen, methyl, ethyl, phenyl and vinyl radicals.
Preferably, the compounds of formulae (I) and (II) are mixed in a
molar ratio (I)/(II) of from 10/90 to 40/60. The titanium compound
is used in an amount of 1 to 10 mol % based on the total of the
organic silicon compounds of formulae (I) and (II). The disilazane
of formula (III) is used in at least equimolar amount to the total
of components (A), (B), and (C).
(7) JP-A 190933/1991 which corresponds to U.S. Ser. No. 07/631,272
EP 434031 A2 and discloses a method for preparing an organic
silazane polymer comprising the steps of:
reacting an organic silicon compound of formula (I): ##STR13##
wherein R is selected from the class consisting of hydrogen,
chloro, bromo, methyl, ethyl, phenyl, and vinyl, R.sup.1 is
hydrogen or a methyl, and X is chloro or bromo, or a mixture of an
organic silicon compound of formula (I) and an organic silicon
compound of formula (II): ##STR14## wherein R.sup.2 and R.sup.3 are
independently selected from the class consisting of hydrogen,
chloro, bromo, methyl, ethyl, phenyl and vinyl, and X is chloro or
bromo, with a disilazane of formula (III): ##STR15## wherein
R.sup.4, R.sup.5 and R.sup.6 are independently selected from the
class consisting of hydrogen, methyl, ethyl, phenyl and vinyl, at a
temperature of 25.degree. to 350.degree. C. in an anhydrous
atmosphere, and
reacting the resulting organic silazane polymer with ammonia,
thereby reducing the residual halogen in the polymer.
(8) JP-A 190932/1991 which discloses a method of preparing a
hafnium-containing silazane polymer comprising reacting
(A) a halogenated organic silicon compound such as those described
above,
(B) a hafnium compound of the following formula (I):
in which X represents chlorine or bromine, and
(C) a disilazane of the following formula (II) ##STR16## in which
R.sub.1, R.sub.2 and R.sub.3 are independently selected from
hydrogen, methyl radical, ethyl radical, phenyl radical and vinyl
radical.
In the present invention, the polycarbosilanes and the
polysilazanes are used as the organic silicone polymers as
described above.
Since the degree of polymerization of the organic silicone polymer
largely affects coating performance, especially coating crack
resistance, the polycarbosilanes should preferably have a number
average molecular weight of about 500 top 5,000, more preferably
from about 600 to about 2,000, most preferably from about 650 to
about 1,200, and the polysilazanes preferably have a number average
molecular weight of about 400 to about 3,000, more preferably from
about 500 to about 2,000, most preferably from about 550 to about
1,200. Below the lower limit of number average molecular weight,
the resulting composition would poorly adhere to substrates. Above
the upper limit, cracks would occur in the resulting coatings which
could be peeled off during subsequent baking.
The organic silicone polymers may be used alone or in admixture of
two or more. Preferably the composition contains about 5 to 50% by
weight, more preferably about 15 to 30% by weight of the organic
silicone polymer based on the total weight of the composition
(organic silicone polymer plus silazane compound plus inorganic
powder). Less than 5% by weight of the organic silicone polymer
would sometimes be too small to provide the composition with
satisfactory heat resistance, adhesion, and coating hardness
whereas more than 50% would sometimes form coatings susceptible to
cracking and peeling after baking.
The organic silicone polymer component is generally converted into
SiC, Si.sub.3 N.sub.4 and the like by subsequent baking of coatings
in an inert gas such as nitrogen and argon. If coatings are baked
in air, then the organic silicone polymer component is converted
into a ceramic material consisting essentially of SiC, Si.sub.3
N.sub.4 and SiO.sub.2, ensuring that the present composition form
fully heat resistant coatings.
A second essential component is a silazane compound which is
preferably selected from tetramethyldisilazane,
hexamethylcyclotrisilazane, and octamethylcyclotetrasilazane alone
or a mixture of two or more.
Preferably the silazane compound is blended in an amount of about 5
to 40%, especially about 10 to 30% by weight of the total weight of
the binding components (organic silicon polymer plus silazane
compound plus inorganic filler). Less than about 5% of silazane
compound would result in less desirable electric insulation whereas
more than about 40% of silazane compound would adversely affect
coating hardness and adhesion.
A third essential component is an inorganic filler which is
preferably selected from Al.sub.2 O.sub.3, SiO.sub.2, Fe.sub.2
O.sub.3, TiO.sub.2, MgO, ZrO.sub.2 -SiO.sub.2, 3Al.sub.2
O.sub.3.2SiO.sub.2, ZnO-MgO, Si.sub.3 N.sub.4, SiC, and BN alone or
a mixture of two or more. The inorganic filler preferably has a
mean particle size of about 0.1 to 30 .mu.m, especially about 1 to
5 .mu.m although the particle size is not critical.
Preferably the inorganic filler is blended in an amount of about 10
to 70%, especially about 30 to 60% by weight of the total weight of
the binding components. Less than about 10% of filler would incur
difficulty of application and pinholes in the coatings whereas more
than about 70% of filler would result in low coating adhesion.
The heat resistant coating composition is applied by dissolving and
dispersing the organic silicon polymer, silazane compound, and
inorganic filler in an organic solvent such as hexane, benzene,
toluene, xylene, and N-methylpyrrolidone. The concentration may
range from 50 to 500 parts by weight of organic solvent per 100
parts of the binding components.
In the first form of the invention, the above-defined coating
composition is first applied to substrates. The type of substrate
is not critical and either metallic or non-metallic substrates may
be used. Preferably, the substrates are pre-treated on their
surface by conventional techniques, for example, by polishing with
sand paper followed by removal of oily values.
Any desired technique may be used to apply the coating composition
to substrates. Exemplary are brush coating, spray coating, flow
coating, dipping, and roll coating. It is preferred to coat the
composition to a (wet) thickness of about 20 to 150 .mu.m,
especially about 30 to 100 .mu.m. Coatings of less than about 20
.mu.m thick are likely to induce pinholes which are detrimental to
corrosion resistance whereas more than 150-.mu.m thick coatings
would partially peel off at the end of baking.
Next, the thus applied coatings are baked after conventional
treatment, for example, drying at room temperature.
Baking is carried out in an atmosphere of ammonia gas or a mixture
of ammonia and an inert gas. Conventional methods carry out baking
in air, which fails to achieve coatings having high electric
insulation at high temperatures for the following reason. When
coating compositions containing polycarbosilane and polysilazane
are baked in air, these components are converted into ceramics of
SiO.sub.2 type which have poor electric insulation at high
temperatures. This can be avoided by baking the coating composition
in an atmosphere of either ammonia gas or a mixture of ammonia and
an inert gas. Then polycarbosilane and polysilazane components are
converted into nitride, Si.sub.3 N.sub.4 which insures high
electric insulation at high temperatures.
Ammonia gas is preferably present in the baking atmosphere at a
concentration of about 10 to 100%, especially about 50 to 100%.
Baking conditions may be properly controlled. Desirable is a
two-step baking procedure including preliminary drying at room
temperature to 300.degree. C., especially 150.degree. to
250.degree. C. for about 5 to 120 minutes, especially about 15 to
60 minutes, and baking at 200.degree. to 1,000.degree. C.,
especially 400.degree. to 800.degree. C. for about 10 to 120
minutes, especially about 30 to 60 minutes.
In the second form of the invention, the above-defined coating
composition is first applied to substrates and baked to form a
first coating layer, and thereafter, a coating composition
containing an organic silicon polymer is applied and baked to form
a second coating layer on the first coating layer.
With respect to the formation of the first coating layer, the
method of applying the heat resistant coating composition to
substrates and the coating thickness are the same as in the first
form. However, baking is preferably carried out in air at a
temperature of about 200.degree. C. or higher for about 15 to 60
minutes. Temperature of lower than 200.degree. C. would result in a
first coating layer having low strength or hardness. Desirable is a
two-step baking procedure including preliminary baking at lower
than 250.degree. C. for about 15 to 30 minutes, and baking at
400.degree. to 700.degree. C. for about 15 to 60 minutes. If
necessary, baking is carried out in an inert gas atmosphere or
another atmosphere.
In this way, there is formed the first coating layer. Since the
first coating layer alone cannot provide satisfactory electric
insulation at high temperatures, an organic silicon polymer base
coating is formed on the first coating layer according to the
second form of the invention in order to provide a second coating
layer having satisfactory electric insulation at high
temperatures.
The second coating layer may be formed by applying an organic
silicon polymer, preferably a solution of organic silicon polymer
in organic solvent, to the first coating layer. The organic silicon
polymer used herein may be polycarbosilane or polysilazane as
previously defined.
The degree of polymerization of the organic silicon polymers is
selected from the standpoint of crack resistance of the resulting
coatings. For example, polycarbosilanes preferably have a number
average molecular weight of about 500 to 5,000, more preferably 600
to 2,000, especially 650 to 1,200. Polysilazanes preferably have a
number average molecular weight of about 400 to 3,000, more
preferably 500 to 2,000, especially 550 to 1,200. These organic
silicon polymers are often used by dissolving them in organic
solvents such as hexane, toluene, benzene, and xylene. The amount
of solvent used varies with the type of organic silicon polymer and
the thickness of coatings although the polymer is often diluted
with the solvent to a concentration of 10 to 70%, especially 30 to
60% by weight. Dipping, spray coating and other conventional
coating techniques may be employed.
Preferably, the organic silicon polymer base coating is about 5 to
150 .mu.m, especially about 10 to 50 .mu.m thick.
After application, the coatings are dried and then baked,
preferably in an atmosphere of ammonia gas or a mixture of ammonia
and an inert gas as in the previous embodiment. Baking in ammonia
gas causes polycarbosilane and polysilazane to convert into
Si.sub.3 N.sub.4 type materials which ensure that the resultant
coatings experience no lowering of electric insulation at high
temperatures. Baking in another atmosphere is less desirable. For
example, baking in inert gas causes polycarbosilane to convert into
SiC plus excess carbon and polysilazane to convert into a SiC and
Si.sub.3 N.sub.4 mixed system, both failing to achieve electric
insulation at high temperatures. Baking in air results in coatings
of SiO.sub.2 material which provide less satisfactory electric
insulation at high temperatures and sometimes low adhesion and low
hardness.
The baking temperature ranges from about 400.degree. to 800.degree.
C., preferably from about 600.degree. to 700.degree. C. Nitriding
does not take place below about 400.degree. C. so that only
coatings of lower hardness are obtained whereas metallic substrates
would be attacked by ammonia gas above 800.degree. C.
According to the method of the invention, there can be formed
coatings which firmly adhere to metallic or non-metallic
substrates, have high heat resistance, that is, withstand
temperatures of higher than about 400.degree. C., and exhibit
excellent other properties including hardness, high-temperature
electric insulation, water resistance, chemical resistance, and
solvent resistance. The invention thus find great utility in
applications of providing corrosion resistant, oxidation resistant
coatings on metallic substrates and heat resistant, dielectric
coatings on conductors.
EXAMPLE
Examples of the present invention are given below by way of
illustration and not by way of limitation.
The organic silicon polymers used in Examples were synthesized by
the following procedures. The first two examples are illustrative
of the synthesis of polycarbosilanes.
REFERENCE EXAMPLE 1
A 5-liter, three-necked flask was charged with 2.5 liters of dry
xylene and 400 grams of metallic sodium and heated to the boiling
point of xylene in a nitrogen gas stream whereby metallic sodium
was dissolved and dispersed. To the flask, 1 liter of
dimethyldichlorosilane was added dropwise over one hour. At the end
of addition, the reaction mixture was heated under reflux until the
reaction was completed. The resulting precipitate was removed by
filtration from the reaction mixture, which was washed with
methanol and then with water, yielding 400 grams of polysilane in
white powder form. Then an autoclave equipped with a gas inlet
tube, agitator, condenser, and distillation tube was charged with
400 grams of polysilane, which was subjected to polymerization
under a pressure of 5 kg/cm.sup.2 G at 450.degree. C. There was
obtained a polycarbosilane, designated Polymer A, having a number
average molecular weight of 1250.
REFERENCE EXAMPLE 2
Reference Example 1 was repeated except that autoclave
polymerization was under a pressure of 5 kg/cm.sup.2 G at
430.degree. C. There was obtained a polycarbosilane, designated
Polymer B, having a number average molecular weight of 900.
The following two examples are illustrative of the synthesis of
polysilazanes.
REFERENCE EXAMPLE 3
A dry 1-liter, four-necked flask equipped with a stirrer,
thermometer, ammonia inlet tube, and deeply cooled condenser was
charged with 850 ml of hexane and then with a mixture of 40.3 grams
of methyldichlorosilane, 7.5 grams of methyltrichlorosilane, and
12.9 grams of dimethyldichlorosilane, and cooled to -20.degree. C.
Excess ammonia gas was admitted into the solution for 4 hours at a
flow rate of 12 liter/hour for reaction. Thereafter, the reaction
mixture was warmed to room temperature while the condenser was
replaced by an ambient cooling condenser so that unreacted ammonia
could escape from the reactor. By removing the ammonium chloride
by-product by filtration and stripping off the hexane solvent,
there was obtained 27.3 grams of liquid silazane.
Next, a 300-ml flask equipped with a stirrer, thermometer, dropping
funnel, and gas inlet tube was charged with 0.2 grams of potassium
hydride and 125 ml of tetrahydrofuran. To the flask was added 27.3
grams of the liquid silazane in 75 ml of tetrahydrofuran at room
temperature through the dropping funnel. Evolution of a large
volume of gas was observed during the addition. The temperature was
raised to 60.degree. C., at which reaction was continued for 2
hours until completion. Then the reaction solution was cooled down.
Addition of 2.5 grams of methyl iodide resulted in a white
precipitate of KI. After the majority of tetrahydrofuran was
removed, 80 ml of hexane was added to the residual white slurry.
The mixture was filtered and the hexane was removed from the
filtrate in a vacuum of 1 mmHg at 70.degree. C., yielding 25.3
grams of a solid silazane polymer, designated Polymer C, having a
number average molecular weight of 1200.
REFERENCE EXAMPLE 4
A dry 2-liter, four-necked flask equipped with a stirrer,
thermometer, gas inlet tube, and condenser was charged with 1.5
liters of toluene and then with a mixture of 149.5 grams (1 mol) of
methyltrichlorosilane and 261 grams (2.4 mol) of
trimethylchlorosilane. Ammonia gas was admitted into the solution
at room temperature for 3 hours at a flow rate of 90 liter/min.
(total NH.sub.3 added 12 mol). With stirring, the reaction mixture
was aged for one hour at room temperature until the reaction was
complete. The ammonium chloride by-product was removed by
filtration and washed with 2 liters of toluene. The toluene was
stripped from the combined filtrate at 120.degree. C. and 30 Torr,
yielding 89 grams of a colorless clear silazane compound having a
molecular weight of 436.
Next, a 300-ml flask equipped with a stirrer, thermometer, and
condenser was charged with 89 grams of the silazane compound. The
reactor was purged with nitrogen gas stream and slowly heated. A
low molecular weight fraction distilled out at a temperature of
270.degree. C. The temperature was further raised to 300.degree. C.
at which the reactor was held for two hours. On cooling the flask,
there was yielded 55 grams of a pale yellow solid, designated
Polymer D, having a number average molecular weight of 1070.
EXAMPLES 1-7 AND COMPARATIVE EXAMPLES 1-2
Coating compositions were prepared in accordance with the
formulation shown in Table 1. To stainless steel pieces of 50
mm.times.50 mm.times.3 mm which had been polished with #240 sand
paper, degreased and cleaned, the coating compositions were applied
to a thickness of 70 .mu.m by means of a bar coater and then dried
at room temperature. The coatings were subjected to preliminary
drying at 250.degree. C. for 30 minutes in air and then baked at
the temperature in the atmosphere both reported in Table 1. The
thus coated steel plates were examined by the following performance
tests. The results are also shown in Table 1.
(1) Coating hardness
The coating was scratched by the pencil scratch test according to
JIS K-5400 to determine pencil hardness.
(2) Adhesion
Adhesion was examined in accordance with JIS K-5400 by scribing the
test piece on the coating surface at intervals of 1 mm, applying
adhesive tape thereto, lifting off the tape, and counting the
number of coating sections left adhered.
(3) Electric insulation
Electric insulation was measured with direct current at 500 V in
accordance with JIS C-1303.
(4) Heat resistance
Heat resistance was examined by heating the test piece in air at
700.degree. C. for 1,000 hours, allowing it to cool down, and
observing whether or not the coating was cracked or separated.
(5) Water resistance
Water resistance was examined by immersing the test piece in hot
water at 80.degree. C. for 1,000 hours and observing whether or not
the coating was cracked or separated.
(6) Alkali resistance
Alkali resistance was examined by immersing the test piece in 10%
NaOH aqueous solution for 1,000 hours and observing the coating for
cracking or separation.
(7) Corrosion resistance
Corrosion resistance was examined by immersing the test piece in
10% HCl aqueous solution for 1,000 hours and observing the coating
for cracking or separation.
(8) Solvent resistance
Solvent resistance was examined by immersing the test piece in
xylene for 1,000 hours and observing the coating for cracking or
separation.
TABLE 1
__________________________________________________________________________
Comparative Example Example 1 2 3 4 5 6 7 1 2
__________________________________________________________________________
Composition (wt %) Binding Organic Polycarbosilane A 20 10 15 20 10
components silicon Polycarbosilane B 20 10 5 polymer Polysilazane C
10 10 Polysilazane D 10 15 15 Silazane Tetramethyldisilazane 10
compound Hexamethylcyclo- 10 20 10 10 10 5 10 20 trisilazane
Octamethylcyclo- 10 tetrasilazane Inorganic Al.sub.2 O.sub.3 40 40
35 50 40 40 filler SiO.sub.2 5 Si.sub.3 N.sub.4 45 BN 45 TiO.sub.2
40 Solvent Xylene 30 20 25 20 30 30 20 Toluene 30 30 Total 100 100
100 100 100 100 100 100 100 Baking Atmosphere NH.sub.3 NH.sub.3
NH.sub.3 N.sub.2 + NH.sub.3 N.sub.2 + NH.sub.3 NH.sub.3 NH.sub.3
air air (1:1) (1:1) Conditions (.degree.C./min.) 700/30 .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. Coating properties
Hardness 9H 6H 6H 7H 7H 6H 8H 3H 2H Adhesion good .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. partial partial separation separation
Insulation (.OMEGA. - cm) 10.sup.13 10.sup.11 10.sup.12 10.sup.12
10.sup.11 10.sup.12 10.sup.12 10.sup.8 10.sup.8 Heat resistance
good .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. partial .rarw.
separation Water resistance good .rarw. .rarw. .rarw. .rarw. .rarw.
.rarw. .rarw. .rarw. Alkali resistance good .rarw. .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. Corrosion resistance good .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. partial .rarw. separation
Solvent resistance good .rarw. .rarw. .rarw. .rarw. .rarw. .rarw.
.rarw. .rarw.
__________________________________________________________________________
As seen from Table 1, the coatings obtained by applying and baking
the heat resistant coating compositions of Examples 1-7 are
excellent in various properties including substrate adhesion,
hardness, insulation, heat resistance, water resistance, and
chemical resistance.
EXAMPLES 8-14 AND COMPARATIVE EXAMPLES 3-4
Coating compositions for the first layer were prepared in
accordance with the formulation shown in Table 2. To stainless
steel pieces of 50 mm.times.50 mm.times.3 mm which has been
polished with #240 sand paper, degreased and cleaned, the first
coating compositions were applied to a thickness of 70 .mu.m by
means of a bar coater and then dried at room temperature. The first
coatings were baked at the temperature in the atmosphere both
reported in Table 2.
Then, coating compositions of the second layer were prepared in
accordance with the formulation shown in Table 2. The second
coating compositions were applied to a thickness of 10 .mu.m by
means of a bar coater, dried, and then baked at the temperature in
the atmosphere both reported in Table 2.
The double coated steel plates were examined by the same
performance tests as before. The results are shown in Table 2.
TABLE 2
__________________________________________________________________________
Comparative Example Example 8 9 10 11 12 13 14 3 4
__________________________________________________________________________
First layer composition (wt %) Binding Organic Polycarbosilane A 20
10 15 20 10 components silicon Polycarbosilane B 20 10 5 polymer
Polysilazane C 10 10 Polysilazane D 10 15 15 Silazane
Tetramethyldisilazane 10 compound Hexamethylcyclo- 10 20 10 10 10 5
trisilazane Octamethylcyclo- 10 tetrasilazane Inorganic Al.sub.2
O.sub.3 40 40 35 50 50 20 filler SiO.sub.2 5 20 Si.sub.3 N.sub.4 45
BN 45 TiO.sub.2 40 Solvent Xylene 30 20 25 20 30 30 20 Toluene 30
30 Baking Atmosphere air .rarw. .rarw. .rarw. .rarw. .rarw. .rarw.
.rarw. .rarw. Conditions (.degree.C./min.) 700/30 .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. .rarw. Second layer composition
(wt %) Organic silicon Polycarbosilane A 50 60 50 50 polymer
Polysilazane C 50 50 50 50 50 Solvent Xylene 50 40 50 50 50 50 50
50 50 Baking Atmosphere NH.sub.3 .rarw. .rarw. .rarw. .rarw.
NH.sub.3 .rarw. air N.sub.2 N.sub.2 Conditions 600/30 .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. 600/30 .rarw. (.degree.C./min.) Coating
properties Hardness 9H 6H 6H 7H 7H 6H 8H 8H 2H Adhesion good .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. partial partial separation
separation Insulation (.OMEGA. - cm) 10.sup.13 10.sup.13 10.sup.13
10.sup.13 10.sup.13 10.sup.13 10.sup.13 10.sup.8 10.sup.5 Heat
resistance good .rarw. .rarw. .rarw. .rarw. .rarw. .rarw. partial
.rarw. separation Water resistance good .rarw. .rarw. .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. Alkali resistance good .rarw. .rarw.
.rarw. .rarw. .rarw. .rarw. .rarw. .rarw. Corrosion resistance good
.rarw. .rarw. .rarw. .rarw. .rarw. .rarw. partial .rarw. separation
Solvent resistance good .rarw. .rarw. .rarw. .rarw. .rarw. .rarw.
.rarw. .rarw.
__________________________________________________________________________
As seem from Table 2, the coatings obtained by applying and baking
the coating compositions twice as in Examples 8-14 are excellent in
various properties including substrate adhesion, hardness,
insulation, heat resistance, water resistance, and chemical
resistance.
Although some preferred embodiments have been described, many
modifications and variations may be made thereto in the light of
the above teachings. It is therefore to be understood that within
the scope of the appended claims, the invention may be practiced
otherwise than as specifically described.
* * * * *